1. Field of the Invention
The invention relates to a process for influencing the properties of combustion residue from a combustion plant, especially a waste incinerator, in which the fuel is burned on a furnace grate and the unmelted and/or unsintered combustion residue which accumulates is returned to the combustion process. Most of the combustion residue originates from the ash content of the fuel and is obtained in the form of grate ash—frequently referred to as slag—in the deslagger. The residues can also include fly ash from the boiler or from the off-gas filtration unit. Grate ash can also contain metal, glass, and ceramic components.
2. Description of the Related Art
A process of this type is known from German Patent No. 102 13 788. In this process, combustion is regulated in such a way that a portion of the combustion residue melts and/or sinters in the combustion bed of the main combustion zone, whereas the unmelted and/or unsintered combustion residues are separated at the end of the combustion operation and returned to the combustion process.
It is also known from European Patent No. 0 862 019 that flue dust can be metered into the high-temperature zone of the combustion furnace, where the temperature is above the melting or sintering temperature of the flue dust. The fly ash, i.e., flue dust, of specific combustion conditions which promote the formation of toxic organic pollutants such as PCDD/PCDF and/or precursor compounds such the precursors of PCDD and PCDF.
These processes take no account of the fact that the return of the combustion residue can have a significant effect on the combustion process. Of particular importance in this regard are the percentage of combustion residue in the fuel mixture and the change in the material composition of the combustion residue.
The return of combustion residue leads, for example, to an increase in the proportion of combustion residue in the fuel mixture and thus to a decrease in the temperature of the combustion bed. Because of the lower combustion bed temperature, the proportion of unmelted and/or unsintered components in the combustion residue increases even more. When these amounts are returned again in turn without regulation in accordance with German Patent No. 102 13 788, for example, the temperature of the combustion bed will continue to drop, which will be disadvantageous.
The material composition of the combustion residue, furthermore, can also change as a result of its return. Unmelted and/or unsintered combustion residue in the form of fine slag fractions have, for example, higher calcium oxide contents and lower iron oxide contents than the average composition of the combustion residue. This means that the average lime content of the combustion residue can increase over time as a result of the return of fine slag fractions as done in accordance with German Patent No. 102 13 788.
The melting and/or sintering process is determined
by the material composition of the fuel and of the returned combustion residue, this composition being in turn the crucial factor which determines the melting temperature and the reactivity during sintering reactions, and
by the combustion conditions, which are the deciding factor with respect to the combustion bed temperature and other essential combustion parameters. The amount of fuel mixture supplied; the point of introduction; the stoking by the grate; and the quantities of air, oxygen, and recycled off-gas and their temperatures determine the combustion conditions.
In the following, a distinction is made between “combustion conditions” and “combustion parameters.” Thus the combustion conditions are the settings which one is able to make or to influence directly by means of control devices. These include, for example, the quantity of fuel mixture supplied (fuel mixture=fuel+returned combustion residue), the point of introduction, and the rate and temperatures at which air, oxygen, and returned off-gas are supplied.
The combustion parameters are defined as the variables which cannot be set directly by means of control devices but rather which are the result of the combustion conditions. These include, for example, the temperature of the combustion bed, the temperature of the combustion chamber, the amount of steam produced, and the O2 content in the off-gas. The composition of the fuel (calorific value, water content, ash content) is also considered a combustion parameter, because it cannot be directly influenced or controlled in the case of waste materials.